DESCRIPTION (taken from the application) We propose to test the role of the transcription factor PDX1 in the control of islet and acinar cell differentiation, and to determine its model of action and its target genes. We have identified the homeodomain partners (PBX1b and MRG1) that form a functional complex with PDX1 in acinar cell lines, but not in Beta-cell lines. By creating a tissue- specific conditional knock-out of the Pbx1 gene that will be deleted only when and where PDX1 is expressed, we will test the role of the complex in acinar and islet cell development (Specific Aim 1). We will test directly of PDX1 to control the formation of the endocrine (Beta-cell) and exocrine (acinar cell) tissues of the pancreas by placing a mouse Pdx1 transgene under the control of a tetracycline-regulated promoter. In mouse embryos and cultured pancreatic rudiments from embryos in which the endogenous Pdx1 gene has been inactivated, the only source of PDX1 will be the regulated transgene (Specific Aim 2). We will identify the genes activated by PDX1 in early development by creating a library of cDNA clones of mRNAs expressed in pancreatic rudiments when they are released from the developmental block caused by the absence of PDX1, but which they are not present in the blocked rudiments prior to PDX1 induction (Specific Aim 3A). We will also identify genes whose promoters normally are bound by PDX1 in Beta- or acinar cell lines (Specific Aim 3B). We will analyze the role of selected PDX1 target genes likely to be important in early pancreatic development or in the differentiation of Beta-cells (Specific Aim 4).